Imported From: https://issues.scala-lang.org/browse/SI-6826?orig=1
Reporter: @paulp
Affected Versions: 2.11.0

@ghik said (edited on May 4, 2013 8:25:12 PM UTC):
I was hit by this bug in 2.10.1 and did a little debugging (used this as an opportunity to delve a bit into compiler internals).

Below is what I found. I don't know if this is of any relevance or tells anything that isn't already obvious to you, Scala compiler guys, but I post this in case I save someone a little debugging time.

In general, this seems to be a problem with lazy resolution of type associated with a Symbol.

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> typeOf[List[_]].typeSymbol.asType.toType.getClass
res0: Class[_ <: reflect.runtime.universe.Type] = class scala.reflect.internal.Types$TypeRef$$anon$6

TypeRef$$anon$6 is a subclass of NoArgsTypeRef, this is a wrong type!
At this point, infos.info of the symbol is an instance of scala.reflect.runtime.SymbolLoaders$TopClassCompleter (retrieved using Java reflection on private fields so it didn't cause any lazy evaluation)

This happened as a result of following call chain:

	at scala.reflect.internal.Types$Type.typeParams(Types.scala:554)
	at scala.reflect.internal.Symbols$Symbol.unsafeTypeParams(Symbols.scala:1468)
	at scala.reflect.internal.Symbols$TypeSymbol.tpe(Symbols.scala:2768)
	at scala.reflect.internal.Symbols$SymbolContextApiImpl.toType(Symbols.scala:101)
	at scala.reflect.internal.Symbols$TypeSymbol.toType(Symbols.scala:2689)

The innermost call (Type.typeParams) returned empty list because it was called on rawInfo which at this point was a lazy type, scala.reflect.runtime.SymbolLoaders$TopClassCompleter. I guess this is something that shouldn't have happened. Based on empty parameter list returned from TopClassCompleter, a NoArgsTypeRef type was associated with the symbol.

OK, let's see what happens next:

scala> typeOf[List[_]].typeSymbol.asType.toType.getClass
res1: Class[_ <: reflect.runtime.universe.Type] = class scala.reflect.internal.Types$TypeRef$$anon$6

Still the same...

scala> typeOf[List[_]].typeSymbol.asType.toType
res2: reflect.runtime.universe.Type = List

Wrong result, that's a string representation of NoArgsTypeRef from before.
At this point however, infos.info of the symbol is an instance of scala.reflect.internal.Types$PolyType. It seems that
something was reinitialized during toString() call on the type.

scala> typeOf[List[_]].typeSymbol.asType.toType.getClass
res3: Class[_ <: reflect.runtime.universe.Type] = class scala.reflect.internal.Types$TypeRef$$anon$5

Now the type seems to be correct. TypeRef$$anon$5 is a subclass of ArgsTypeRef.

scala> typeOf[List[_]].typeSymbol.asType.toType
res4: reflect.runtime.universe.Type = List[A]

Now that the symbol has correct type, everything's fine.

@paulp said:
"I guess this is something that shouldn't have happened."

Right, that's why it's called "unsafeTypeParams".

Calling toString sometimes leads to normalize being called (that is a bug, which I might have fixed in master, can't remember) which can force the info.

It's a big, complicated thing.